Broadly, this invention relates to an apparatus and method for detecting the presence of certain relatively low molecular weight target components in a liquid. In one aspect, this invention relates to an apparatus and method for detecting the presence and concentration of relatively low molecular weight components in liquid hydrocarbons. In another aspect, this invention relates to an apparatus and method for determining the Reid Vapor Pressure (RVP) of a liquid hydrocarbon. In a more detailed aspect, this invention relates to an improved method and apparatus for determining, separately or cumulatively, the concentration of propane, butane and pentane components in crude oil.
In recent years the public has become increasingly concerned about excess ozone formation. While federal regulations aimed at limiting vehicle emissions already exist, public concern has prompted the consideration of even lower limits for allowable gasoline vapor pressure in areas having poor air quality. Lower vapor pressure gasoline emits less hydrocarbons and is therefore less likely to contribute to ozone formation. Gasoline's vapor pressure can be lowered by reducing the concentration of low molecular weight components in the gasoline.
Propane, butane and pentane are the most common low molecular weight components of gasoline. The amount of these "light end" components which must be removed by oil refiners depends on the concentration of light ends in crude supplied to refineries. Crude containing excessive concentrations of light ends is undesirable because refining costs to produce low vapor pressure gasoline are higher when such crude is used. Besides higher refining costs, refiners who pay crude oil prices for light ends contained in crude oil may incur changes for propane, butane and pentane that are significantly higher than the market value of these components.
Crude oil producers and pipeline operators are also interested in knowing the concentration of light components in crude. Producers separate light components from crude before selling it to a carrier. Efficient control of the separation process requires accurate on-line analysis of the RVP of crude oil. Pipeline operators strive to maintain the concentration of light ends in crude at levels low enough to be acceptable to refiners. It is important that carriers prevent crude with excessive concentrations of light ends from entering pipeline systems.
The determination of the concentration of light end components in hydrocarbons is necessary for efficient process control and for accurate custody transfer pricing. To be useful for process control, an apparatus and method for determining the concentration of light ends in liquid hydrocarbons must be adapted to field environments and should be able to generate on-line results. To be useful for custody transfer, such apparatus and method must be suitable for transport to and operation at multiple field locations. Prior to this invention, no such apparatus or method has been available which meets these requirements.
At present, apparatus and methods for determining concentrations of light ends in hydrocarbons include various gas stripping techniques in combination with gas chromatographs or pressure analyzers. U.S. Pat. No. 3,150,516 (Linnenbom et al.) discloses, for example, a system for extracting dissolved gases from a liquid sample and subsequently analyzing the extracted gases using a gas chromatograph. While such instrumentation may be operable for on-line analysis, it is primarily intended for laboratory use. Further, extraction and chromatography typically take 10-20 minutes per sample. The '516 system is clearly unsuitable for remote field applications where portability and durability are critical for utility.
U.S. Pat. No. 3,446,077 (Sanford et al.) discloses, for example, a sampling system which is capable of providing vapor samples from liquid streams. The '077 system does not encompass gas analysis. Further, the apparatus for extracting gas from a liquid comprises a stripping column wherein extraction occurs by means of precisely maintained pressure and flow conditions.
Commercially available Reid Vapor Pressure analyzers include the Model No. 44770 Reid Monitor, manufactured by Precision Scientific of Chicago, Ill. This device is capable of measuring the RVP of distillate fuels, but not crudes, in accordance with ASTM procedures. RVP measurements are made under conditions of constant temperature and carefully controlled vapor to liquid ratios using an absolute pressure bellows and two pneumatic transmitters.
Another commercially available device for measuring the absolute vapor pressure of hydrocarbon compounds, including crude oil and distillate fuels, is the Precision Scientific Model 41351 Absolute Vapor Pressure Monitor. This device continuously measures vapor pressure under conditions of constant temperature and precisely controlled pressure drop across a venturi-type ejector system. It may be calibrated to measure RVP for certain hydrocarbon compounds.
Prior art methods and apparatus for vapor pressure and light ends analysis of liquid hydrocarbons are deficient because they are large, complex devices which require gas chromatographs or other sensitive equipment which tends to be fragile and not readily suited for field applications. Further, prior art methods and apparatus cannot be readily transported to remote locations. Still further, prior art methods are relatively time consuming. For instance, full analysis of a hydrocarbon compound may take 10-20 minutes. Poor response time reduces the effectiveness of prior art methods for on-line applications. Another shortcoming of the prior art is the inability to analyze for light ends in crude oil. Yet another shortcoming is the inability to distinguish between different light end components commonly found in crude oil.
It should also be pointed out that different varieties of crudes having, for instance, different viscosities, gravities, sulfur contents, etc., present different analytical problems. For instance, if a stripping technique is used, different crudes are going to respond differently to the same stripping gas being contacted at the same conditions and in the same amounts, i.e., temperature, pressure, and flow rate.
To illustrate the need for field-ready devices, particularly devices which are portable, it is useful to consider methods commonly used at present for monitoring the quality of crude oil at an inlet point of a pipeline or the loading end of a vehicular transport system. Typically, crude oil is collected in tank batteries servicing local production operations. Prior to reaching custody transfer tanks, certain impurities, including light ends, are removed from the crude. In order to minimize operating costs, producers generally remove only the amount of impurities necessary to meet safety and crude oil quality requirements of regulatory agencies and transport operators.
Very often, crude oil producers establish area offices whose personnel are responsible for the production of crude in oil fields covering many acres. Such area offices are responsible for the operation of various, sometimes numerous, tank batteries scattered throughout their territory. Production economics limit not only the level of staffing for oil fields, but also the quantity and cost of equipment, such as analytical devices for monitoring crude oil quality at custody transfer points. Typically such analysis equipment is available only at a central laboratory located at and operated by the area office. Staffing and equipment limitations prevent close monitoring of crude oil. Transportation operators must contend with similar limitations.
Problems and inefficiencies with present crude analysis methods are made clear in the following example. When an oil producer completely fills a custody transfer tank and desires to sell or otherwise provide such crude oil to the associated transport operator, the quality of the crude oil becomes a matter of interest to both parties: the transporter because he does not want to accept delivery of substandard crude oil, and the producer because he wants to confirm that tendered crude oil either does or does not meet specifications, such as RVP requirements. When a transporter is asked to take delivery of crude, it is common practice to obtain samples of the tendered crude oil for analysis. The crude storage tank is typically sealed while the samples are carried to a laboratory for analysis. If analysis indicates acceptable crude oil quality, the transporter must return to the tank, check then break the seal, and initiate custody transfer. It is not unusual for this procedure to take 24-48 hours. If analysis indicates unacceptable crude oil quality, such as an excessively high RVP, the problem is communicated to the producer, corrective action is taken, and the analysis process is repeated, including inherent inefficiencies in obtaining and transporting samples, and taking custody of the crude oil. Meanwhile, continuing production can be delayed or impeded because tankage is tied up.
While the oil industry has tolerated and adjusted to such problems and inefficiencies, there has long been a need for improved apparatus and methods for determining the concentration of light ends in hydrocarbon liquids. Further illustration of this long felt but unmet need exists with respect to custody transfer pricing, refining and gas separation activities.
The present invention provides for a relatively simple method and apparatus for determining the concentration of target components in a liquid which can be carried out relatively quickly in the field regardless of the type of liquid analyzed. The present invention dispenses with the use of fragile columns as employed in gas chromatographic techniques.